1. Field of the Invention
The present invention relates generally to drilling systems and more particularly to a system of drilling boreholes having a measurement-while-drilling ("MWD") system wherein drilling and formation data and parameters determined from various downhole measuring devices are transformed downhole into selected parameters of interest or "answers" which are telemetered to the surface or stored downhole for subsequent retrieval or both. In an alternate embodiment, measurements are depth-correlated, utilizing depth measurements made downhole for improving accuracy of the measurements and the parameters of interest. The measurements and/or parameters are also correlated with stored reference data for providing additional information pertaining to the drilling operations and the formation characteristics. The system also is adapted to determine the drill bit location relative to the desired drilling path and to adjust the drilling activity downhole based on such determination.
2. Background of the Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill pipe having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the "mud motor"). Pressurized drilling fluid (commonly known as the "mud" or "drilling mud") is pumped into the drill pipe to rotate the drill motor and to provide lubrication to various members of the drill string including-the drill bit. As required the drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes.
A plurality of downhole devices are placed in close proximity to the drill bit to measure formation properties, downhole operating parameters associated with the drill string and to navigate the drill bit along a desired drill path. Downhole devices, frequently referred to as the measurement-while-drilling ("MWD") devices, are typically coupled between the drill bit and the drill pipe along with the mud motor, kick-off device and stabilizers. For convenience, all such devices are collectively sometimes referred herein as the "downhole subassembly." The MWD devices typically include sensors for measuring downhole temperature and pressure, an inclination measuring device for determining the inclination of a portion of the drill string, a resistivity measuring device to determine the presence of hydrocarbons against water, and devices for determining the formation porosity, density and formation fluid conditions.
Prior to drilling a borehole, substantial information about the subsurface formations is obtained from seismic surveys, offset wells, and prior drilled boreholes in the vicinity of the current borehole. The borehole is then usually drilled along a predetermined path based upon such prior information. During the borehole drilling, the downhole subassembly transmits information about the various downhole parameters, which are typically analyzed and correlated with other parameters at the surface to decide whether the drilling path needs to be adjusted. To adjust the drilling path, the drill string is usually retrieved from the borehole and then certain mechanical devices, such as kick-off subassemblies and stabilizers, are adjusted to alter the drilling direction. Stopping the drilling operation and retrieving the drill string to adjust the drilling direction results in great expense. Additionally, surface-measured downhole depth of the drill bit is typically utilized to take corrective actions. Surface-measured depth readings rely on the drill pipe length, which over several thousand feet may have an error of several feet (15 to 50 feet) from the true location, which is highly undesirable, especially for horizontal drilling through relatively narrow formations. Thus, it is desirable to have a drilling system which provides more accurate measure of the depth of the downhole subassembly and means for adjusting the drilling direction without retrieving the drill string from the borehole.
The downhole subassembly usually transmits information about the various downhole parameters to the surface by an uplink telemetry via the mud column in the drill string or electromagnetic means. The current telemetry systems such as the mud-pulse telemetry systems are capable of transmitting typically one bit per second, which greatly limits the ability to transmit a vast amount of useful information about the downhole formations and downhole conditions to the surface during the drilling operation.
To accurately determine the properties of the formations along the borehole, such as porosity, permeability, hydrocarbon saturation and other geophysical properties and the borehole profile, the drilling activity is intermittently stopped, the drill string is retrieved and wireline logs are obtained by traversing wireline tools through the borehole. The wireline tools contain a set of downhole devices such as resistivity devices, porosity and permeability measuring devices and acoustic devices. Such devices transmit a vast amount of data relating to the formations and the downhole conditions via a high transmission rate telemeter system to the surface, where the data is transformed into certain parameters of interest, which parameters are then utilized to aid in the drilling of the borehole and to determine the formation lithology, producibility of the pay zones, etc. The wireline systems provide a method for conveying the devices in the borehole and means for transmitting data at very high data rates.
Current wireline systems contain multiple sensors and complex processing algorithms to determine formation properties along the borehole. Examples include electromagnetic sensors comprising multiple transmitters and multiple receivers which measure attenuation and phase shift of the transmitted signals as they traverse the formation. Acoustic sensors which measure attenuation, phase shift and the full wave form of acoustic signals traversing the formation and borehole are also used. Nuclear sensors are used to measure the natural gamma ray energy spectrum of the formation which is indicative of shale content, shale type and other parameters of interest. Nuclear sensors comprising chemical neutron or isotopic gamma ray sources and neutron or gamma ray detectors are used to measure a plurality of geophysical parameters. Pulsed neutron sources and gamma ray accelerators are used in other types of nuclear sensors. All of the aforementioned sensors used in the wireline tools are data intensive. When such measurements are made simultaneously with a single pass of a multiple sensor wireline device along the borehole, massive amounts of raw data are generated per depth interval of borehole traversed.
With some wireline systems, raw sensor data are transmitted to the surface of the earth over the logging cable for subsequent processing to obtain the multiple parameters of interest. As examples, current wireline telemetry systems using seven conductor electrical logging cable can telemeter data to the surface at a rate of 500 kilobits to 1000 kilobits per second. Use of fiber optic cables substantially increases the data transmission rate. Such wireline telemetry systems have large telemetry bandwidths which enable the use of multiple sensors and transmission of the data to the surface for processing.
However, in boreholes in which the pressure of the well is above atmospheric pressure at the surface, the logging cable must pass through a pressure-containing device known in the art as a "lubricator." The cross sectional area of current multiple conductor and fiber optic cables is such that the lubricator cannot contain surface well pressures of several thousand psi and still permit the cable to move freely through the lubricator. Single conductor cables have smaller cross-sectional areas which allow the lubricator to maintain pressure control and also allow the cable to move freely through the lubricator. Therefore, smaller diameter single-conductor cables are usually used in such high pressure wells. Telemetry bandwidths of single conductor wireline systems are substantially lower than those comprising seven conductors or fiber optic cables.
Simultaneous measurements using multiple, data-intensive sensors can generate amounts of raw data which exceed the telemetry capacities of single conductor wireline systems. As a result, the raw data is sometimes compressed before being telemetered to the surface, which results in a loss of vertical resolution of the measurements and/or a degradation in accuracy of the measurements. Vertical resolution and accuracy can be preserved by correlating multiple measurements downhole in such wireline systems.
Wireline logs from existing wells are frequently correlated to select locations and borehole profiles of subsequent wells. For example, log correlations often define the presence of faults and aid in the delineation and mapping of fault blocks. Log correlations can also reveal anomalies such as localized structures or "lenses" which might act as traps for hydrocarbons. However, such wireline correlations often do not reveal critical structural aspects of the field as will be illustrated and further discussed in subsequent sections of this disclosure. Correlation of measurements during drilling of the borehole can provide more accurate measure of such anomalies. It is therefore desirable to determine parameters of interest downhole and correlate such parameters of interest with prior well logs during the drilling operation.
Seismic data are frequently used in developing existing oil and gas fields. Seismic data are usually the prime source of information upon which decisions are based in choosing locations for exploratory or "wild cat" wells, but seismic data are also used in the development of existing fields. The correlation of well log data and seismic data can be used to detect structural anomalies which would go undetected with conventional well log correlation methods. The spatial resolution of seismic measurements is poor when compared to wireline measurements. Although well log and seismic correlations are used to select locations and target zones of development wells, poor spatial resolution presents a problem in defining the target formation with the accuracy and precision required by the driller. Correlation of downhole-computed parameters of interest with seismic data during drilling of the borehole can address some of these problems. The results may be stored downhole for later retrieval and/or selectively transmitted uphole during drilling of the borehole.
Thus, there is a great need to determine various parameters of interest downhole during the drilling of boreholes because massive data generated by the sensors cannot be transmitted uphole during the drilling operation. As noted earlier, wireline logs are typically made intermittently during the drilling operation and such logging requires that the drill string be removed from the borehole prior to logging. Logging after completion of the drilling operation often reveals that the target formation or formations have been missed. In addition, unexpected zones such as high pressure formations or salt zones, can be encountered during the drilling operation. These formations and zones can add costly delays to the drilling operation and endanger drilling personnel and equipment. Also, damage to the borehole can occur during logging and drilling rig time and logging equipment time is wasted during stand-by periods for each operation.
Many of the above-noted problems can be overcome by measuring various formation-evaluation and drilling parameters during the actual borehole drilling operation. Formation evaluation via measurements-while-drilling (MWD) logs combined with offset wireline logs and seismic data can provide, in real time, information on anomalies, such as fault planes or formation lenses. Such measurements can also indicate to the driller that high-pressure formations or salt zones are being penetrated, thereby giving the driller time to take remedial steps, such as adjusting the weight and salinity of the drilling fluid, before these zones adversely affect the drilling operation. Real-time measures of drilling dynamics data provide the driller with information concerning the efficiency of the drilling operation. Furthermore, borehole directional information, combined with real-time formation evaluation parameters, offset wireline log data and seismic data can be extremely useful in assisting the driller in reaching the targeted zone of interest. These and other applications involving the correlation of offset wireline data, seismic data and any other type of reference data with measurements made while drilling will be discussed further herein.
These MWD systems provided directional information and a limited number of formation evaluation type measurements. In the past decade, additional sensors and services have been added. In many respects, the sophistication of the sensors is comparable to their wireline counterparts in spite of the harsh environment experienced in using such sensors in the drilling environment. Current MWD systems do not combine multiple sensor measurements because current MWD telemetry does not have the capacity to simultaneously transmit a plurality of full acoustic wave forms or gamma ray energy spectra or electromagnetic wave attenuation and phase shift data, or a combination thereof, to the surface for processing to determine parameters of interest at intervals sufficient to obtain the required vertical resolution of the penetrated formations. The simultaneous transmission of drilling management sensor information such as directional information, weight on the drill bit, and other non formation evaluation type measurements further overloads current MWD telemetry transmission rates, which are of the order of 2 to 60 bits per second. Furthermore, it is not feasible to store copious amounts of raw downhole sensor data for subsequent retrieval and processing due to relatively limited storage capacity of current MWD systems.
MWD means for making multiple formation and borehole evaluation type parametric determinations comparable to current wireline measurements require the computation of the desired parameters downhole, and the transmission of the computed parameters of interest to the surface. By using downhole computational means and methods, the transmission requirements are reduced by orders of magnitude in that only "answers" are telemetered rather than raw data. This type of downhole computation is also applicable to other types of non-formation evaluation type measurements such as signals indicative of the operational characteristics of the downhole equipment as well as measurements indicative of drilling direction and efficiency.
There are numerous references in the prior art directed toward the measurement of formation parameters while drilling and the use of these measurements to "steer" the drill bit within a formation of specified characteristics. An exemplary system is disclosed in U.S. Pat. No. 5,163,521 to Randal H. Pustanky et al. One basic technique comprises the transmission of measured formation parameters to the surface in real-time thereby allowing the driller to compare measured and targeted formation parameters and to manually adjust the direction of drilling based upon this comparison by the use of directional commands from the surface. A second basic technique comprises the continuous comparison of measured and target formation parameters downhole and the automatic adjustment of the drilling direction based upon these comparisons. U.S. Pat. No. 5,332,048 to Lance D. Underwood et al teaches the measurement of geological parameters while drilling, the use of a downhole microcontroller which is preprogrammed with a desired range of formation characteristics or with the desired borehole inclination or target area, the continuous comparison of measured and preprogrammed formation characteristics, and the adjustment (either automatically or by commands from the surface) of the drilling direction based upon these comparisons. The formation evaluation features of this and other references directed toward "geosteering" are rather fundamental in that they are designed to identify the formation that is being penetrated (e.g. sand or shale) for steering purposes rather than to perform a detailed analysis of the formation.